This invention relates to toning systems in magnetographic printing and more specifically to improvements in the arrangement of a toning system for transferring dry particulate magnetic toner to magnetic latent images on a magnetic medium, taking into particular consideration the development of high quality images and efficiency of toning.
In electrophotography, the transfer of a proper amount of dry magnetic toner to the photoconductor substrate bearing the electrostatic latent image is accomplished by carefully balancing the electrostatic forces of the latent image against the magnetic forces of the applicator exerted on electrically charged toner particles. For example, a decrease in the magnetic forces of the applicator causes an increase in the amount of toner transferred to the latent image. Notwithstanding, arrangements have been proposed whereby the so-called lead magnet of the applicator, i.e. the magnet positioned to be primarily responsible for governing the toner transfer to the medium, has been located as close as practically possible to the photoconductor substrate and oriented with a pole face thereof fully facing the substrate to provide a perpendicular magnetic field relative to the latter.
In magnetography, one of the more effective techniques of transferring toner to a magnetic latent image carried on a medium is described in U.S. Pat. No. 4,197,811 to Nelson. In this technique the magnetic latent image is subjected to a free flowing amount of dry magnetic toner particles, and subsequently the excess toner deposited on the magnetic latent images is removed by properly controlled forces of air flowing over the toned images. Such a system is capable of providing high quality toned images and does not require any magnetic fields to bring the toner into contact with the magnetic medium, and thus there is not danger of the magnetic fields of the applicator apparatus adversely affecting the latent magnetic images. However, for commercial utilization, this so-called free flow toner applicator has the disadvantage that involved and time consuming procedures are needed for replacing the magnetic substrate or medium. Also, toner gets behind tape and must be removed.
Another technique of transferring magnetic toner particles to a magnetic latent image is described in U.S. Pat. No. 3,945,343 (Berkowitz). In this magnetically controlled technique, apparently magnetic toner is transferred to the latent image by carefully controlling the magnetic forces, exerted on the toner particles, of the image and the applicator while the direction of a reduced applicator magnetic field provided by a pair of spaced magnets of the same magnetic polarity is maintained substantially perpendicular to the plane of the latent image to reduce the interaction of the two magnetic fields that could result in weakening (or destruction) of the magnetic latent image. It can be appreciated that in such a system the toner applicator may not interfere with the function of replacing the used magnetic substrate, as compared with the arrangement of the aforementioned U.S. Patent. However, transfer of a proper amount of toner to the magnetic latent image, without image degradation, for high quality print remains of substantial concern, especially when considering toning efficiency as one of the prime goals. That is, when the strength of the magnetic field of the applicator is adjusted upward for a proper amount of toner transfer (i.e. to ensure essentially no over-toning) the magnetic latent images on the magnetic substrate or medium tend to become weakened, which causes degradation in the quality of the toner image. While the problem can be addressed by selecting a magnetic recording medium with substantially higher values of coercive force, it can be appreciated, however, that such changes in the properties of the magnetic recording medium will create other difficulties in the recording functions.
In yet another known magnetic roll applicator arrangement, the pair of magnets of the applicator closest to the magnetic medium are spaced apart to provide a reduced magnetic field and yet are provided with a double-sided ramp member therebetween to mechanically urge the toner particles in a perpendicular direction toward and relative to the plane of the medium. Such an arrangement, however, does not satisfy the conditions of high quality toning at commercially desirable high speeds of operation due to toner starvation and excessive air borne toner escaping the toning zone.
Therefore, what is needed and would be useful is a toner applicator for transferring dry magnetic toner particles to a moving magnetic latent image with efficiency and without causing image degradation or machine contamination, to be utilizable in commercial high quality printing systems, and such is a principal object of this invention.